LONDON — It is always satisfying to prove your doubters wrong and, in the case of Jeremy Corbyn, the left-wing leader of Britain’s opposition Labour Party, there were an awful lot of them.

Written off as a hapless loser 12 months ago at his last party conference, Mr. Corbyn can expect a triumphant reception at this year’s event, which was to begin on Sunday.

Last year, he was widely depicted as an unreconstructed Marxist and a political dinosaur, destined to lead Labour to electoral extinction. Now, Mr. Corbyn is seen, even by some opponents, as a prime minister in waiting — an astonishing transformation for a political veteran who spent decades on the fringes of British politics.

At 68, Mr. Corbyn is in many ways a British version of Senator Bernie Sanders of Vermont, except further left, to the point of superannuation, many once believed. In June, though, he exceeded expectations with a clever general election campaign that revealed him to be a personable, if not downright charming, candidate.

His unexpected ability to connect with voters, particularly young ones, coupled with a building opposition to Britain’s decision to leave the European Union, or Brexit, helped him deprive Prime Minister Theresa May of the landslide victory she had expected when calling the vote. With that also went her parliamentary majority, and her authority over an increasingly fractious Conservative Party.

So, not surprisingly, Mr. Corbyn has a smile on his famously bearded face these days.

“He’d been moldering away in musty old rooms for 20 or 30 years, talking to very small audiences,” said Steven Fielding, a professor of political history at Nottingham University. “And now this is someone who could be the next prime minister, and whose supporters think he would introduce the most transformative program since 1945.”

“Yes, he’s going to be enjoying that,” he added.

While Mr. Corbyn’s journey from zero to hero has been remarkable, Professor Fielding says, the party conference in Brighton, England, will bring a new question into focus: whether Labour can shift from a defensive strategy primarily intended to keep control of his party to an offensive one that could take Labour to power.

To do so, Mr. Corbyn may need to evolve from the leader of an insurgent left-wing social movement to the head of a party that can garner broad support.

Since his election triumph, there has been an upturn in his image as a down-to-earth politician who rides a bike, makes his own jam and is a multiple winner of Parliament’s beard of the year competition. When he spoke at the Glastonbury rock festival this summer, the young people in the crowd chanted his name.

While many of Britain’s predominantly right-wing newspapers remain hostile to Mr. Corbyn’s agenda, their tone has changed. Where the news media once pointed to his failure to sing the national anthem as evidence of his unsuitability for high office, recent articles have debated things like whether Mr. Corbyn is turning from vegetarianism to veganism (apparently he is not, though he is eating more vegan food).

The news media “were so amazed that the election did not turn out in the way they expected, and that Corbyn did better than predicted,” Professor Fielding said, “that they have been slightly falling over themselves” and into a “perspective which has gone slightly beyond reality.”

After all, Mr. Corbyn did not win the election, even when up against an opponent, Mrs. May, who proved to be one of the worst top-level campaigners in years.

To win a general election, he will have to reach out to new voters, and “whether he is in a position to go and win a majority in Parliament is an open question,” said Mark Wickham-Jones, professor of political science at the University of Bristol. “We know that the party can mobilize at election time, but whether that sense of Labour as a social movement can become a sense of Labour as a structured political party, we don’t know.”

At the conference in Brighton, which runs through Wednesday, so-called Corbynistas may vent their anger at centrist and right-wing lawmakers. But there is little sign of a concerted effort to deprive right-wing Labour lawmakers of their seats. That could mean that now that Mr. Corbyn’s leadership is secure, his priorities are shifting toward seeking control of the country, rather than the party.

That may be hard. By the time of the next election, the Conservatives are likely to have replaced Mrs. May with a more popular figure, and Mr. Corbyn and his agenda will probably face tougher news media scrutiny.

Mr. Corbyn is a lifelong critic of the European Union who nevertheless campaigned last year for Britain to remain in the bloc. But he did so without enthusiasm, prompting some pro-Europeans to blame him for the vote to leave.

Labour’s paradox is that, while most of its voters and its young backers wanted to remain in the Union, many working-class supporters in the party’s heartlands deserted it, attracted to the “Leave” campaign’s anti-immigration rhetoric.

Mr. Corbyn managed to appeal to both sides in the general election by keeping his Brexit policy fuzzy. With negotiations on withdrawal underway, Labour argues that it has accepted the outcome of the referendum but wants to retain close economic ties to the European Union in order to protect jobs.

How this might be achieved, while restricting the free movement of European workers, as Labour has hinted it would do, has not been explained.

So Labour is sticking with “constructive ambiguity,” opposing Mrs. May’s negotiating strategy, staying vague about what it would do instead, and planning to blame the Conservatives if the outcome is a mess.

So far, that strategy seems to be going to plan, as members of Mrs. May’s cabinet continue to fight a vicious internal battle over Brexit and to position themselves to succeed her.

In fact, it is working so well that William Hague, a former leader of the Conservative Party who accused Mr. Corbyn last year of taking arguments “back to the 1980s,” recently issued a unity plea to warring colleagues.

If they continue as now, Mr. Hague said, there will be little point in Conservatives discussing who is going to lead them, “because Jeremy Corbyn will be prime minister, sitting in Number 10.”